Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellowness, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Therefore, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, the most representative method is a method using an episulfide compound as described in Patent Document 1.
Moreover, it has been studied to achieve a high refractive index, and a composition consisting of one or more types of inorganic compounds selected from inorganic compounds having a sulfur atom and/or a selenium atom and an episulfide compound described in Patent Document 2 has been proposed.
Furthermore, since white turbidity may occur when polymerizing and curing a composition comprising sulfur and an episulfide compound, proposals regarding the improvement of transparency have been made in Patent Documents 3-5.
However, in the case of lenses having a large central thicknesses called plus-power lenses, the occurrence of white turbidity was not successfully inhibited even by the above-described proposals. Since the plus-power lenses concentrate transmitted light, even slight reduction in transparency tends to be confirmed macroscopically, and it can be said that the form of the plus-power lenses tends to allow white turbidity to easily occur. Further, since compositions are used for optical materials, when white turbidity occurs after curing, all becomes defective products, resulting in heavy losses. Accordingly, a technique which makes it possible to predict whether or not white turbidity will occur after curing and to determine quality at a stage before curing has been desired.